Cryptosporidium parvum is an important global cause of persistent and chronic diarrhea. In young children, particularly in underdeveloped countries, cryptosporidiosis is clinically more severe and is associated with more intestinal inflammation than infection in adults. Little is understood about the mucosal "innate" immune response to C. parvum. Gamma delta (gammadelta) T cells are an important T cell population in young children and in the intestinal tract. In response to infection at the intestine, T cells appear to expand in the periphery and selectively "home" back to the intestinal mucosa, via a gut-specific homing receptor (alpha4beta7). Intestinal gammadelta T cells appear to modulate recovery of the epithelium, respond to intestinal "stress" and may have important cytolytic function, especially in killing of infected intestinal epithelial cells (and possibly CD4+ alphabeta T cells). This K08 proposal seeks to train the investigator in classic and molecular/genetic immunology techniques and to apply this basic science knowledge to study mucosal immune responses to cryptosporidiosis in children, especially those in the developing world. The proposal hypotheses that gammadelta T cells are important in the innate immune response to cryptosporidiosis, are activated in response to C. parvum via the T cell receptor with necessary co-stimulation by the gut-specific MHC-I molecule (MIC-A), and kill infected epithelial cells by cytolysis and apoptosis. This information will then be applied to the study of gamma delta T cells from a small population of Bangladesh, children with acute cryptosporidiosis to demonstrate in vivo relevance. The primary mentor of this proposal is a full professor of Immunology with expertise in gammadelta T cells and with a laboratory skilled in the proposed research assays. Co-mentors are two experienced researchers, well known to the applicant, with depth in cryptosporidiosis and international research on enteric infections, respectively. The academic plan outlines structured formal coursework and conferences and provides depth to the laboratory training. In the research proposal, Aim 1 establishes whether the activation of the gammadelta cells is via the TCR and requires co-stimulation with the MHC-I like molecule MIC-A. In this aim, cloned gamma and gamma chains of the T cell receptor are transfected to a T cell receptor negative cell line and evaluated for function. Aim 2 evaluates the cytotoxicity of gammadelta via Fas-FasL and perforin and confirms whether cytotoxicity is antigen-dependent. Finally, to establish in vivo relevance, Aim 3 examines the gammadelta T cells from children with cryptosporidiosis, establishes whether these gammadelta cells (carrying the alpha4beta7 homing receptor) can be extracted from peripheral lymphocytes and if these clones have activation and cytolytic functions similar to the in vitro data. These data will advance our understanding of the mucosal immune response to cryptosporidiosis, which will lead to further advances in prevention and therapy.